Typically it is quite difficult (read: expensive) to get hold of official ISO standards. It is by chance that I discovered the draft version of ISO 13579-1 on ‘Industrial furnaces and associated processing equipment — Method of measuring energy balance and calculating efficiency — Part 1: General methodology’ on the website of AFNOR, the French body of standards. The draft is open for comments as part of a public hearing process.

The draft standard ISO 13579-1 talks about energy balance (‘bilan énergétique’). Part 4.2 shows a sample Sankey diagram and explains that this is a tool that allows to represent the in and out flows of energy (‘outil permettant de représenter le flux d’entrée et de sortie de l’énergie’). In section 9 f on reporting the draft standard recommends that the report shall include a Sankey diagram (‘Il convient que le rapport de mesure du bilan énergétique contienne … Diagrammes de Sankey.’).

Didn’t have the original English version at hand, so I hope this is pretty much what it says in French.